1. Field of the Invention
The present invention relates to systems for supplying gas, in particular compressed natural gas, such as for example methane, for internal-combustion engines, of the type comprising:                a plurality of electromagnetically controlled injectors, associated to the various cylinders;        a distribution manifold or rail communicating is with said injectors;        a reservoir for supply of the rail, where pressurized gas is accumulated; and        a pressure-reducing valve set in connection between the-reservoir and the aforesaid rail.        
2. Prior Art
A supply system of the known type referred to above is illustrated in FIG. 1 of the annexed plate of drawings. In said figure, the reference number 1 designates the electromagnetically controlled injectors associated to the various cylinders of the engine, which are supplied with pressurized gas by a distribution manifold or rail 2. The reference number 3 designates a gas cylinder, which functions as a reservoir, in which pressurized gas, for example methane, is accumulated. The outlet of the gas cylinder 3 is connected via a pipe 4 to the rail 2. Set in series in the pipe 4 are: a safety valve 5, constituted by a solenoid shutoff valve designed to block the outlet of the gas cylinder 3; a pressure sensor 6; and a pressure-reducing valve 7. The reference number 8 designates a sensor of the pressure in the rail or distribution manifold 2.
In the case, for example, of a methane supply system, the initial pressure of the methane inside the gas cylinder 3, when the latter is full, is in the region of 200 bar. Said pressure of course drops as the gas cylinder 3 empties, until a minimum value in the region of 20 bar is reached.
At the same time, the electromagnetically controlled injectors 1 are able to operate at sensibly lower gas pressures, normally lower than 10 bar. The purpose of the valve 7 is precisely to bring the pressure of the gas to a suitable value for proper operation of the injectors 1. In the practical case, currently used pressure-reducing valves bring the pressure of the gas in the pipe 9 downstream of the pressure-reducing valve 6, which takes the gas to the rail 2, to a pressure value which oscillates (as the pressure of the gas coming from the pipe 4 varies) between approximately 6.3 bar and 8.5 bar.
The present invention relates, in particular, to systems for supplying gas of the type illustrated above, in which the pressure-reducing valve comprises:                a valve body, with an inlet connector connected to the reservoir and an outlet connector connected to the rail;        a restricted passage defined inside the valve body for communication between the aforesaid inlet connector and the aforesaid outlet connector;        an open/close element for control of the communication through said restricted passage;        means for return of the open/close element tending to keep the open/close element in an open condition; and        a piston member, movable inside the valve body, for controlling said open/close element, said piston member being subject to the pressure of the gas downstream of the aforesaid restricted passage.        
FIG. 2 of the annexed plate of drawings illustrates a pressure-reducing valve of a known type used in supply systems of the type referred to above. The example illustrated relates to the case of a valve that provides two successive stages of pressure reduction set in cascaded fashion. The body of the valve is designated by the reference number 10. The number 11 designates the inlet connector, designed to be connected to the pipe (FIG. 1) through which the gas coming from the reservoir under pressure 3 flows, whilst the reference number 12 designates the outlet opening, in which there is designed to be mounted the connector for connection to the pipe 9 that takes the gas at reduced pressure to the rail 2 (FIG. 1). The connector 11 defines an inlet passage 13 that communicates with the outlet 12 through a series of passages made inside the body 10, as will be defined further in what follows. Set in said series of passages is a restricted passage 14 associated to the first stage of the valve. The gas that enters the valve through the inlet passage 13 arrives at the restricted passage 14 passing through a filter 15 and an electromagnetically controlled safety shutoff valve. The solenoid valve 16 comprises a solenoid 17 that is able to recall an anchor 18 into a retracted position, in which an open/close shutoff element 19 is disengaged from a respective valve seat, leaving a passage 20 that converges into the restricted passage 14 free. The restricted passage 14 gives out onto a spherical surface, functioning as valve seat, which co-operates at the front with an open/close element 21 constituted by a seal element mounted at a free end of a stem 22 of a piston member 23. The latter has a bottom head (as viewed in FIG. 2) of widened diameter, which is slidably mounted, with the interposition of a seal gasket 24, within a cylindrical liner 25 fixed to the body of the valve. A helical spring 26 is set between the bottom head of the piston member 23 and a fixed cup 27. The spring 26 tends to keep the piston member 23 in its end-of-travel position downwards (illustrated in the drawing), in which the bottom head of the piston element 23 is in contact with a bottom element 28 for closing the cylinder liner 25 and in which the open/close element 21 is set at a distance from the outlet of the restricted passage 14, so that in said condition the gas that arrives at the restricted passage 14 from the inlet passage 13 can pass into a chamber 29 that is set downstream of the restricted passage 14, after undergoing a consequent pressure drop. From the chamber 29, the gas flows via an intermediate passage 30 to a second stage of the valve, which is identical to what has been described above from a functional standpoint, via which the gas finally reaches the outlet opening 12. In what follows, said second stage of the valve will not be further illustrated, since it corresponds, as has been said, to the first stage. To return now to the structure and to the operation of the first stage of the pressure-reducing valve, the gas that arrives in the chamber 29, in addition to flowing towards the outlet through the passage 30, also reaches a chamber 31 facing the opposite end of the piston member 23 via an axial passage 32 made through the piston member 22 and through radial holes provided in the wall of the stem of the piston member. The chamber 33, in which the spring 26 is set, is in communication with the external atmosphere through holes 25a provided in the wall of the cylinder liner 25. Consequently, the seal gasket 24 performs the function of preventing the gas present in the chamber 31 from being able to leak into the chamber 33 and from there come out into the external atmosphere. A similar function is performed by a seal gasket 34 provided in a position corresponding to a central hole of the fixed cup 27 functioning as guide for the sliding movement of the stem 22 of the piston member 23. Also said gasket in fact prevents the gas present in the chamber 14 from possibly passing into the chamber 33 and from there into the external atmosphere. The seal gaskets 24 and 34 are designed obviously taking into account the fact that they are set between surfaces in relative movement, i.e., they are gaskets of a dynamic type. Static gaskets 35, 36, constituted by seal rings made of elastomeric material, are instead set between the closing element 28 and the bottom end of the cylinder liner 25 and between the fixed cup 27 and the body of the valve.
In operation, the gas coming from the inlet passage 13 passes initially straight into the chamber 29 through the restricted passage 14, undergoing a pressure reduction through the solenoid valve 16 in its initial opening phase, and is thus sent at reduced pressure to the passage 30, from which it passes to a second pressure-reducing stage, or directly to the outlet of the valve (in the case of the valve being a single-stage one). As the pressure in the chamber 29 increases, however, said pressure is also communicated to the chamber 31 located at the opposite end of the piston member 23. On account of the greater effective area at the surface of the head of the piston member 23 facing the chamber 31, when the pressure in the chamber 31 reaches the calibration pressure value, i.e., the reduction pressure of the first stage, the pressure of the chamber 31 tends to bring about raising (as viewed in the drawing) of the piston member 23 against the action of the spring 26 until it brings about closing of the open/close element 21 against its seat. The open/close element thus remains closed until the pressure in the chamber 29, and consequently in the chamber 31, drops back to a value such that the spring causes opening of the open/close element. There is thus brought about a continuous oscillation of the open/close element between the open condition and the closed condition, which keeps the pressure in the pipe 30 downstream of the first reduction stage within a required range of values. As has already been said, the operation described above is repeated a second time at the second stage of the valve, in the case where the valve is a dual-stage one, as in the example illustrated in the figure, whilst the gas that arrives at the pipe 30 is sent directly to the rail in the case of a single-stage valve.
Drawbacks of the Prior Art
In the known valves of the type described above, it is necessary for the dynamics of variation of the regulated pressure not to exceed ±10%, in order to guarantee proper operation of the injection system. With reference to FIG. 1 of the annexed plate of drawings, the pressure sensor 8 sensitive to the pressure in the rail 2 sends its signal to an electronic control unit C, which receives also the output signal from the pressure sensor 6 set immediately downstream of the reservoir 3, and controls the injectors 1 and in particular their opening time according to the supply pressure.
In order to limit the sensitivity of the regulated pressure to the pressure of the reservoir and to the flow rate, it is consequently necessary to adopt, in the valve 10 illustrated in FIG. 2, a spring 26 of high load, which accordingly has a large size. This entails the adoption of the geometry illustrated in FIG. 2, with the consequent need to provide two seal gaskets 24, 34 of a dynamic type. It should also be noted that the structure of the cylinder liner 25, within which the piston member 23 is slidably mounted, is at times subject to deformations following upon tightening of the screws 37, which fix the closing element 28 and the cylinder liner 25 to the body of the valve. Possible deformations of the structure increase the risk of leakage of gas towards the outside. Of course, in the case of a dual-stage valve, the aforesaid drawbacks are twice as many.
Basically then, the valve of a known type proves cumbersome, not altogether reliable with respect to the risk of leakage of gas into the external atmosphere, and finally also has a relatively complicated and costly structure. In addition to this, the valve described above does not enable an optimal response during the transient regimes. There is finally the risk of a deterioration of the load of the spring over time, with consequent variation of the regulated pressure.